I do agree that Preeti is a weakling but honestly if there is any person in that house who even slightly cares for Sumo other than nanu(who is not in a condition to bear such shocking revelations) its daboo and Preeti. Since daboo is a kid so we are left with only Preeti. Rest all are just worthless be it mama-mami or masi-mausa. Nirmala is the biggest panauti so no benefit telling her. The best part about telling Preeti is that Sumo has sacrificed Shravan for Preeti and Pushkar's relation so this burden will definitely bother Preeti till she reveals it. Also she is not emotionally that strong to keep this truth to herself for long and she being in Malhotra house now and being a live witness to Shravan's pain will take no time to spill the beans. You may call it Sumo's weak moment or the tendency to share secrets with your peers/siblings...But CVs have chosen the perfect person to know this truth. JUST THE PERFECT!! From here this truth will reach all the right places - Pushkar, Kamini and finally Shravan 😃